The present invention is directed, in general, to electronic systems that incorporate surface acoustic wave (SAW) devices and, more specifically, to electronic systems that include SAW-based low phase noise oscillators that are tunable over comparatively wide ranges.
Many electronic systems contain oscillator circuits that generate high-frequency reference signals that are used both internally and externally by the electronic system. For example, the reference signal may be used as a high-speed CPU clock in a data processing device or as a high-frequency local oscillator (LO) carrier signal in a communication device, such as a base station transceiver in a wireless network. The high-frequency reference signal may also be used as a calibration reference in testing and measurement systems.
Generally, it is highly desirable for an oscillator circuit to exhibit low phase noise characteristics. This is usually accomplished by using resonators or tuned circuits that have a high Q-factor. Because quartz crystals and SAW devices exhibit much higher Q-factors than ordinary LC circuits, oscillator circuits that use quartz crystals and SAW devices as frequency determining elements inherently exhibit much lower phase noise than conventional LC oscillator circuits.
Additionally, many electronic systems require that an oscillator circuit be electronically tunable over some required frequency range. It is preferred that the tuning range be comparatively wide and highly linear in the tuning range. Frequently, a varactor diode or other capacitive element is used in conjunction with LC oscillator circuits to provide a tuning capability. The varactor diode interacts with the inductance of the LC oscillator circuit to generate a variable output frequency.
Unfortunately, however, coupling a varactor diode or other capacitive element to a high Q-factor frequency controlling element, such as a SAW device, does not result in a tunable, low phase noise oscillator circuit. As a practical matter, the same electrical characteristics that cause a high Q-factor frequency controlling element to produce good phase noise performance also limit the tuning range of that circuit.
To overcome this limitation, oscillator circuits have been implemented that couple together two or more SAW devices in order to achieve a wider tuning range and low phase noise. However, the use of multiple SAW elements increases the complexity and the cost of these oscillator circuits.
There is therefore a need in the art for improved oscillator circuits that exhibit low phase noise and linear tuning characteristics. In particular, there is a need in the art for a tunable oscillator circuit that uses only a single one-port SAW resonator as a frequency determining element. More particularly, there is a need for a voltage controlled SAW oscillator (VCSO) that exhibits low phase noise and linear tuning across a comparatively wide tuning range (on the order of +/xe2x88x92400 PPM).
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a low phase noise oscillator circuit having an input port and an output port that receives a frequency tuning signal on the input port and generates a tunable high-frequency output signal on the output port. Accordingly, in an advantageous embodiment of the present invention, the oscillator circuit comprises: 1) a tuning element coupled to the input port and having a variable capacitance responsive to the frequency tuning signal; 2) a first inductor coupled in series with the tuning element; 3) a SAW resonator coupled in series with first inductor; 4) a second inductor coupled in parallel with the SAW resonator; 5) a negative resistance generating circuit coupled to the SAW resonator, wherein, at a first oscillation frequency of the oscillator circuit, a reactance looking into the input port is approximately zero and a resistance looking into the input port is negative.
According to one embodiment of the present invention, the tuning element comprises at least one varactor diode.
According to another embodiment of the present invention, a frequency domain curve of the reactance has a steep slope at the operating frequency.
According to still another embodiment of the present invention, modifying the variable capacitance causes the reactance looking into the input port to equal approximately zero at a second oscillation frequency.
According to yet another embodiment of the present invention, the slope of the reactance is approximately linear at the operating frequency.
According to a further embodiment of the present invention, an operating frequency of the oscillator circuit may be varied across an operating range having an upper frequency limit and a lower frequency limit, wherein the frequency domain curve of the reactance has a steep slope across the operating range.
According to a still further embodiment of the present invention, the frequency domain curve of the reactance across the operating range is approximately linear.
According to a still further embodiment of the present invention, the SAW resonator has a resonance frequency and the first inductor causes the first oscillation frequency of the oscillator circuit to match the resonance frequency.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled. in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated with xe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.